Control of CMP removal rate uniformity by selective control of slurry temperature

ABSTRACT

A CMP machine ( 100, 200, 300 ) and/or process that uses selective heating of the slurry ( 160 ) to improve uniformity. A temperature control mechanism ( 110 ) is used to heat and/or cool slurry ( 160 ) applied to a selected area of the pad or belt ( 120, 220, 320 ). Heating in the selected area improves the removal rate in that area, whereas cooling decreases the removal rate in that area. For example, heating along the perimeter of the pad ( 120, 220 ) improves the removal rate at the perimeter of the semiconductor wafer ( 150 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The following co-pending application is related and hereby incorporated by reference:

[0002] Serial No. TI-29332

[0003] Filing Date

[0004] Inventor(s) Swanson

FIELD OF THE INVENTION

[0005] The invention is generally related to the field of semiconductor processing and more specifically to chemical-mechanical polishing semiconductor wafers.

BACKGROUND OF THE INVENTION

[0006] Chemical-mechanical polishing (CMP) for planarizing semiconductor wafers during fabrication is becoming more and more common. A CMP system generally consists of a polishing pad, wafer carrier, and slurry. As a wafer carrier positions a semiconductor wafer against the polishing pad, slurry is added between the polishing pad and the wafer. The wafer, the pad, or, more typically, both are moved to planarize the surface of the wafer. CMP employs both a mechanical removal of material (due to the physical abrasion of the polishing pad and slurry particles against the surface of the wafer) and a chemical removal (etch) of material (due to the chemical components of the slurry).

[0007] Three basic types of architecture are currently being manufactured. The first type is a rotary polisher. In a rotary polisher, the platen (and the polishing pad it holds) has a radius that is slightly larger than the diameter of the semiconductor wafer. Both the platen and the wafer are typically rotated. The second type of CMP machine is an orbital polisher. In an orbital polisher, the platen diameter is slightly larger than the wafer diameter. The wafer is rotated, but the pad is not. The wafer's center orbits around an axis of rotation offset slightly from the pad center. The third type of CMP machine is a linear belt polisher. In a linear belt polisher, a continuously fed belt is moved over the platen. The wafer is rotated during polishing.

[0008] The planarization uniformity on many polishing machines is difficult to control. This can be due to such process irregularities as pad conditioning, down force, and slurry delivery. Hence, achieving good planarization across a wafer is difficult. This is especially true for copper CMP, which is currently under development.

SUMMARY OF THE INVENTION

[0009] The invention is an improved CMP machine and/or process that uses selective control of the slurry temperature to improve uniformity. Slurry is applied to the polishing pad/belt at several locations. At least one slurry location includes a temperature adjustment mechanism to adjust the slurry temperature for a more uniform removal rate. For example, heating slurry applied along the perimeter of the pad and/or cooling slurry applied near the center of the pad may improve the removal rate uniformity by increasing the removal rate at the perimeter of the semiconductor wafer and/or decreasing the removal rate near the center of the wafer.

[0010] An advantage of the invention is a CMP machine and/or process having improved planarization uniformity.

[0011] This and other advantages will be apparent to those of ordinary skill in the art having reference to the specification in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In the drawings:

[0013]FIG. 1 is a top view of a rotary polisher modified to include a selective control of the slurry temperature according to the invention;

[0014]FIG. 2 is a top view of an orbital polisher modified to include selective control of the slurry temperature according to the invention; and

[0015]FIG. 3 is a top view of a belt polisher modified to include a selective control of the slurry temperature according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0016] The invention will now be described in conjunction with three separate CMP machine architectures. It will be apparent to those of ordinary skill in the art that the invention may be applied to other machine architectures as well.

[0017] It is known that the copper removal rate during polish increases as the pad and slurry temperature rises. This is due to the fact that the chemical component of the CMP process is thermally activated. In the invention, the temperature of the slurry applied to selective areas of the pad is adjusted to improve the uniformity across a wafer during CMP. For example, heated slurry may be applied to selective areas of the pad that correspond to areas of the wafer having a low removal rate. Alternatively, or additionally, cooled slurry may be applied to selective areas of the pad that correspond to areas of the wafer having high removal rate. By balancing out the removal rate across a wafer, uniformity is improved.

[0018] In copper CMP, the removal rate is lower near the edges of a wafer (˜2-5 cm inset from the edge of the wafer by a few mm) than near the center of the wafer. In order to improve the removal rate uniformity across the wafer, heated slurry may be applied to the area of the polishing pad that polishes more of the edge of the wafer than the center. The heated slurry, in turn, increases the removal rate in that area making it more uniform across the wafer.

[0019] New slurries specifically for shallow trench isolation and copper CMP are currently under development. An important feature of these new slurries is that they have a highly non-linear removal rate vs. down force relation. The result is that high features on a wafer are polished down significantly faster than low areas. The higher the differential between the high and low site removal rate (RR), the better. This way dishing of wide features, such as Cu bond pads is minimized. The conventional way that this is achieved through slurry design is to build in two competing mechanisms: passivation, and chemical etch, each with there own reaction rates. The sequence of events during CMP is basically the following:

[0020] 1. The surface is passivated after immersion into the fluid

[0021] 2. The pad, along with the slurry abrasive, removes the passivation layer. Since the pad pressure on the low or recessed sites is less than that on the high areas, the passivation RR is less there.

[0022] 3. Chemical attack by the etchant component of areas lacking passivation. Since passivation RR is less on low areas, the etch rate will also be less, and the overall material RR is also less. The etchant component may be a chemical, which merely converts the surface to a softer material that is very easily removed by the pad/abrasive combination, effectively increasing the material RR in that manner.

[0023] 4. The overall surface tends toward a planar topography, as desired.

[0024] The idea behind using an increased or decreased slurry temperature is to increase the reaction rate of the etchant in step 3 above. Since all chemical reactions are thermally activated, changing the temperature should vary the rates in step 3.

[0025] It should be noted that since the slurry heats the pad, the pad may be softened. Such softening may increase dishing on those areas of the wafer. This is an effect to be balanced against the need to achieve uniform removal across the wafer.

[0026]FIG. 1 shows a rotary polisher 100 modified to include selective control of the temperature of the slurry applied to selective areas of the polishing pad 120. In a rotary polisher 100, the platen 140 has a radius that is slightly larger than the wafer 150 diameter. Platen 140 is used to hold pad 120. Wafer 150 is held against polishing pad 120 and rotated by a wafer carrier (not shown).

[0027] Slurry 160 is applied to the polishing pad 120 at several sites. Two such sites are shown in FIG. 1. Temperature control mechanism 110 is located at one or more of the slurry application sites. Temperature control mechanism 110 heats or cools the slurry immediately before the slurry is applied to the polishing pad 120. The volume of temperature control mechanism 110 depends on the rate slurry is applied to the wafer and the length of time required to change the temperature of the slurry. It is expected that a 100 ml volume is sufficient. The slurry may be heated for selective areas of the polishing pad 120 where an increased removal rate is desired. Alternatively, or additionally, the slurry 160 may be cooled for selective areas of the polishing pad where a decreased removal rate is desired. For example, in copper CMP the removal rate is lower near the edge of the wafer. Therefore, to improve this non-uniformity, the slurry 160 a applied to a peripheral area of the polishing pad 120 may be heated since this area contacts the outer portions of the wafer 150. The slurry 160 b, applied to a more central site, may alternatively or additionally be cooled to decrease the removal rate in that area.

[0028] In operation, slurry 160 is applied to the pad 120 from several locations. FIG. 1 shows two slurry dispense locations. Additional slurry dispense locations may be included. Wafer 150 is pressed against pad 120 with the desired downforce and both the pad and wafer are rotated. The arrows on the pad and wafer indicated rotation direction. The slurry 160 a at selected locations may be heated using temperature control mechanism 110 to improve the removal rate in those areas. The slurry is heated above room temperature to a temperature as high as 30-40° C. Alternatively, or additionally, the slurry 160 b at other selected locations may be chilled using temperature control mechanism 110 to decrease the removal rate in those areas. The slurry may be chilled below room temperature to a temperature as low as 5° C. For copper CMP, the slurry 160 a applied to the periphery of the polishing pad 120 is heated and/or the slurry 160 b applied near the center of the polishing pad 120 is chilled. The slurry 160 b is intended to reach the center of wafer 150 and the slurry 160 a affects only the wafer edge. Lines with arrows extending from the dispense points show the approximate path of the slurry. Slurry 160 b spends more time circulating around the wafer carrier, hence penetrating more deeply under the wafer 150. The slurry 160 a is expected to have a greater impact on the outer edge as slurry 160 a is in contact with the wafer carrier for far less time. The pad under the outer edge may be warmed up due to the slurry and further contribute to an increased removal rate. The result is a more uniform removal rate across the wafer 150.

[0029] A number of different slurries are used in CMP. The stability of the slurry at various temperatures should be considered when setting the temperature. One danger is that particulates may congeal out of suspension and collect on the inner surfaces of the heater. It is known that the abrasive material of the slurry will collect on the surface of a container where an ultrasonic transducer is mounted outside to measure fluid column height. Apparently, the ultrasonic energy induces agglomeration. So heater construction and operation is important in avoiding agglomeration. Any thermal spiking of the heater would raise the heater surface temp much higher than the set temp, causing a high thermal gradient through the slurry.

[0030]FIG. 2 shows an orbital polisher 200 modified to include selective temperature control of the slurry applied to the polishing pad 220. In an orbital polisher 200, the platen 240 has a diameter that is slightly larger than the wafer 150 diameter. Platen 240 is used to hold pad 220. Wafer 150 is held against polishing pad 220 and rotated by a wafer carrier (not shown).

[0031] As with the rotary polisher, temperature control mechanism 110 selectively controls the temperature of slurry 160 applied at various locations of the polishing pad 220. Temperature control mechanism 110 is not shown in FIG. 2, but would be placed below polishing pad 220. Slurry 160 is applied through holes in the polishing pad 220 and may be heated or chilled in selected areas just prior to application to the polishing pad. For example, in copper CMP the removal rate is lower near the edge of the wafer. Therefore, to improve this non-uniformity, the slurry 160 a applied to the periphery 230 of the polishing pad 220 may be heated since this area contacts the outer portions of the wafer 150. Alternatively or additionally, the slurry 160 b applied to a more central location 232 of the polishing pad 220 may be chilled to decrease the removal rate nearer the center of wafer 150. Although only one slurry dispense point is shown in each zone for simplicity, each zone actually contains multiple slurry dispense holes.

[0032] Temperature control mechanism 110 should be located as close to the point of application to polishing pad 220 as possible. Temperature control mechanism 110 could include an array of heaters 112 to heat the slurry or an array of chillers. Chillers generally operate by circulating cooled H₂O-ethylene glycol mixtures. Alternative heating and cooling mechanisms will be apparent to those of ordinary skill in the art.

[0033] In operation, slurry 160 is applied to the pad 220 through holes in the pad 220. Wafer 150 is pressed against pad 220 with the desired downforce and the wafer 150 is rotated. The pad 220 does not rotate, but the center orbits around an axis of rotation. The slurry applied to selected areas of the pad 120 are heated using temperature control mechanism 110 to improve the removal rate in those areas and/or slurry applied to other areas may be chilled to decrease the removal rate in those areas. FIG. 2 shows three temperature zones (230, 232, and 234). At least two zones are required, but additional zones may be included. For copper CMP, the slurry 160 a applied to the periphery 230 of the pad 220 is heated and/or the slurry 160 b applied to a more central location 232 of the pad 220 is chilled. The slurry 160 c applied to an intermediate zone 234 may have a temperature between the temperature of slurry 160 b and 160 a. This results in a more uniform removal rate across the wafer 150.

[0034] A linear belt polisher 300 modified to include slurry temperature control mechanism 110 is shown in FIG. 3. Linear belt polisher 300 comprises a continuously fed belt 320. Wafer 150 is held against polishing belt 320 and rotated by a wafer carrier (not shown).

[0035] Slurry temperature control mechanism 110 may be used to selectively heat slurry 160 applied to selected areas of the polishing belt 320 where an increased removal rate is desired. For example, in copper CMP the removal rate is lower near the edge of the wafer 150. Therefore, to improve this non-uniformity, the slurry 160 a is heated and applied to a portion of the polishing belt 320 where the slurry is in contact with the wafer carrier for a short time since this area contacts the outer portions of the wafer 150. Alternatively, (or additionally) slurry temperature control mechanism 110 may be used to selectively cool slurry 160 b applied to selected areas of polishing belt 320 where there is a longer contact with the wafer carrier. This results in the slurry penetrating more deeply under the wafer.

[0036] Temperature control mechanism 110 may be located as close to the point of slurry application as possible. For heating, either an electrically operated heat exchanger, or a hot water heat exchanger may be used. A chiller may be used to cool the slurry, by using a heat exchanger very similar to the hot water system. The semiconductor industry commonly uses chillers or heaters that operate by circulating H₂O-ethylene glycol mixtures, which are at the desired temperature. In fact, the same unit may be used to heat or chill on demand.

[0037] In operation, slurry 160 is applied to the pad 320 at multiple locations. Wafer 150 is pressed against pad 320 with the desired downforce, the wafer is rotated and the continuously fed belt 320 is moved. The direction of rotation and belt feed are indicated in FIG. 3. The slurry applied to selected areas of the pad 320 may be heated using heating mechanism 110 to improve the removal rate in those areas. For copper CMP, the slurry 160 a is heated and applied to the pad 320 so that minimal contact with the wafer 150 occurs (i.e., the right edge of the belt). Thus, the heated slurry 160 a affects the removal rate near the edge of wafer 150. Alternatively, or additionally, the slurry 160 a is chilled and applied to the polishing belt such that the slurry is in contact with the wafer edge for a longer period of time (i.e., the left side of the belt). Thus, the slurry 160 b penetrates more deeply under the wafer 150 to affect the removal rate nearer the center of wafer 150. This results in a more uniform removal rate across the wafer 150.

[0038] While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments. 

In the claims:
 1. A method of fabricating an integrated circuit, comprising the steps of: placing a wafer against a moving polishing pad while adding a first slurry to a first location and a second slurry to a second location wherein a temperature of said first slurry is higher than a temperature of said second slurry.
 2. The method of claim 1 , wherein said polishing pad is a polishing belt.
 3. The method of claim 1 , wherein said temperature of said first slurry is between room temperature and 40° C.
 4. The method of claim 1 , wherein said temperature of said second slurry is between 5° C. and room temperature.
 5. The method of claim 1 , wherein said second slurry spends a higher percentage of time near a center said wafer than said first slurry.
 6. The method of claim 1 , wherein said second slurry penetrates further under said wafer than said first slurry.
 7. A method of fabricating an integrated circuit, comprising the steps of: providing a partially fabricated wafer to a wafer carrier of a chemical-mechanical polish (CMP) machine; moving a polishing pad of said CMP machine; adding slurry to a surface of said polishing pad from at least two dispense points, wherein there is a slurry temperature gradient between said at least two dispense points; and placing said wafer against said surface while said polishing pad is moving.
 8. The method of claim 7 , wherein said polishing pad is a polishing belt.
 9. The method of claim 7 , wherein a first of said at least two dispense points is located near a center of said polishing piece and a second of said at least two dispense points is located near an edge of said polishing piece.
 10. The method of claim 9 , wherein a temperature of said slurry from said first dispense point is less than a temperature of said slurry from said second dispense point.
 11. The method of claim 7 , wherein slurry from a first of said at least two dispense points penetrates further under the wafer than slurry from a second of said at least two dispense points.
 12. The method of claim 7 , wherein slurry from a first of said at least two dispense points has a lower temperature than slurry from a second of said at least two dispense points.
 13. A chemical-mechanical polish (CMP) machine comprising: a platen; a polishing pad located over said platen; a wafer carrier for holding a wafer against said polishing pad; and at least two slurry dispense outlets, wherein at least one of said slurry dispense outlets contains a temperature control mechanism for establishing a slurry temperature gradient between said at least two slurry dispense outlets.
 14. The CMP machine of claim 13 , wherein said a first of said at least two slurry dispense outlets is located nearer a center of said polishing piece and a second of said at least two slurry dispense outlet is located nearer an edge of said polishing piece. 